This invention relates to a power amplifier formed in a semiconductor integrated circuit device, and more particularly to a high-efficiency power amplifier that amplifies audio signals on plural channels at a high efficiency and is suitable for, for example, a monolithic power amplifier IC (integrated circuit) provided in a car stereo unit.
Many power amplifiers provided in audio units, including car radios and cassette players, generally operate as class B amplifiers or as class AB bridge circuits.
It is well known that the efficiency (the ratio of the output power to the input power from the power supply) of a class B amplifier with a sine wave signal varies from approximately 0% when the output is almost zero to 78.5% (=.pi./4) when the output is the largest. In the case of normal musical sound signals, it is frequently used in regions with very low efficiencies. For example, when the efficiency of a class B amplifier is 20%, this means that the amplifier will dissipate about five times the audio output (100%.div.20%) thermally as power consumption.
Power amplifier ICs provided in car audio units are often provided in a limited space. Therefore, it is difficult to install heat-dissipating devices, such as large heat-radiating fins. Another problem is that high heat damages the semiconductor devices in audio units or shortens their service life.
In this connection, as recent car audio units have been requiring greater power, assuring more channels, and providing more multimedia services, there have been greater demands for high-efficiency, less-heat-dissipation power amplifier systems.
Various efforts have been made to develop audio power amplifier ICs that operate from the same power with a lower heat dissipation (a low power consumption). For example, the following have been developed and produced on a commercial basis: class D amplifiers by high-frequency switching PWM (pulse-width modulation) and class G amplifiers that switch between power supplies according to the signal level (as described in reference 1: B. Murari, F. Bertotti, G. A. Vignola, "Smart Power ICs: Technologies and Applications," New York, Springer, 1955, pp. 416-419), class H amplifiers that lift up the potential of the power supply temporarily when the signal level is great (as described in reference 2: Philips Semiconductor, 40W Car Radio High Power Amplifier TDA1560Q DATA SHEET, 1996, May 14), and class SB amplifiers that switch between a bridge and a single circuit according to the signal level (as described in reference 1, pp. 419 to 423).
Although class D amplifiers have as high an efficiency as about 95%, they have fatal disadvantages for use in ordinary cars in that they require not only a large LC low-pass filter for removing carriers in high-frequency switching but also a large, heavy shielding case for preventing unnecessary radiation.
Class G amplifiers have about twice the efficiency of normal class B or class AB amplifiers in amplifying musical sound. They, however, need plural power supply rails and therefore are not suitable for use in cars with a practically single power supply rail.
Class H amplifiers have about twice the efficiency of normal class B or class AB amplifiers as class G amplifiers. They are at a disadvantage in that they need additional power amplifiers for lifting up the power supply potential and plural large-capacity capacitors.
Class SB amplifiers have the merit of achieving a similar efficiency to that of class G amplifiers by means of a single power-supply rail. They, however, have disadvantages in that they need additional bi-directional high-speed power switches and delay circuits for adjusting the timing of switching and that they make no improvement in the efficiency at the reversed-phase signals. In addition, they are limited to pairs for every two channels from the viewpoint of circuit configuration. Therefore, at non-pair channel, it is impossible to desire the effect of improving the efficiency by using a configuration with three channels or an odd number of channels more than three.